IT was as big an upset as there's been in any football decider, but grand finals are by definition unfair.

Football teams have gone into the big one unbeaten, have defeated their grand-final opponents three times in previous meetings, only to lose the one that mattered most.

Bonnyrigg White Eagles went into Monday's Football NSW premier league decider after having won the competition by 15 points, only for the previously vanquished Marconi to beat them 2-0 in the one that mattered most.

Unfair?

Perhaps, but Marconi are worthy champions.

The clue that the Stallions were galloping home came in their previous finals meeting where Bonnyrigg struggled to win 1-0, and that through a penalty.

The peaking Marconi didn't need penalties on Monday, they had sharpshooting Nathan Elasi, who scored both goals, and against his former club.

Marconi were simply the best.

Bonnyrigg created chances to win but didn't convert them; Elasi took his, in the 65th and 94th minutes.

That was the difference.

He was Robbie Slater Medal winner as man of the mate.

"Obviously, the elation of having won hasn't sunk in yet, as well as getting the medal, but it was all down to the hard work and efforts of our team and the coaching staff," said the 22-year-old former Melbourne Victory and Australian Youth player, who still dreams of an A-league return.

"We played as one today and this showed on the pitch."

Coaching head man Luke Casserly soaked up the frenzied celebrations, then dropped his bomshell — no coaching next season.

"I think the past few weeks the major difference has been that we have had most of our players back from injury and most players available for selection," he said.

"This has given us the confidence and belief that was needed to approach this match with the correct mentality."

Casserly, who was with Marconi when they lasted had success, in the old NSL, cited family and work commitments for his break.

After such heartbreak, Bonnyrigg coach Brian Brown could only offer the immortal words of next season.

"It has been a long time since I have won a grand final, though I think today our opponent played a very good match," was the gracious concession.

"I think the loss today will drive us forward. We have already started planning for next season."